Magnetoresistance effect element

ABSTRACT

A magnetoresistance effect element has a first ferromagnetic metal layer, a second ferromagnetic metal layer, and a tunnel barrier layer that is sandwiched between the first and second ferromagnetic metal layers, and the tunnel barrier layer has a spinel structure represented by a composition formula AGa 2 O x  (0&lt;x≤4), and an A-site is a non-magnetic divalent cation which is one or more selected from a group consisting of magnesium, zinc and cadmium.

This is a Continuation of application Ser. No. 15/554,066 filed Aug. 28,2017, which is a National Stage Application of PCT/JP2016/060034 filedMar. 29, 2016, which claims priority on Japanese Patent Application No.2015-071411, filed on Mar. 31, 2015, the content of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a magnetoresistance effect element.

BACKGROUND ART

Giant magnetoresistance (GMR) elements formed of a multilayer filmconsisting of a ferromagnetic layer and a non-magnetic layer, and tunnelmagnetoresistance (TMR) elements using an insulating layer (a tunnelbarrier layer or a barrier layer) as a non-magnetic layer have beenknown. In general, TMR elements have higher element resistance than GMRelements, but a magnetoresistance (MR) ratio of the TMR elements ishigher than that of the GMR elements. The TMR elements can be dividedinto two types. One type is related to TMR elements using only atunneling effect using an effect of soaking-out of a wave functionbetween ferromagnetic layers. The other type is related to TMR elementsusing coherent tunneling using conduction in a specific orbit of anon-magnetic insulating layer where tunneling is carried out when theabove-described tunneling effect is caused. TMR elements using coherenttunneling have been known to obtain a higher MR ratio than TMR elementsusing only tunneling. The coherent tunneling effect is caused in a casewhere both of the ferromagnetic layer and the non-magnetic insulatinglayer are crystalline and an interface between the ferromagnetic layerand the non-magnetic insulating layer is crystallographicallycontinuous.

Magnetoresistance effect elements are used for various purposes. Forexample, magnetoresistance effect-type magnetic sensors have been knownas magnetic sensors, and magnetoresistance effect elements determinecharacteristics of a reproducing function of hard disk drives. Magneticsensors have a system that detects, as a resistance change of amagnetoresistance effect element, an effect that a magnetizationdirection of the magnetoresistance effect element is changed by anexternal magnetic field. Highly anticipated devices aremagnetoresistance change-type random access memories (MRAM). MRAMs arememories that read magnetoresistance as digital signals of 0 and 1 byappropriately changing ferromagnetic magnetization directions of twolayers to parallel or antiparallel directions.

LITERATURE Patent Documents

[Patent Document 1] Japanese Patent No. 5586028

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2013-175615

Non-Patent Documents

[Non Patent Document 1] Hiroaki Sukegawa, a [1] Huixin Xiu, TadakatsuOhkubo, Takao Furubayashi, Tomohiko Niizeki, Wenhong Wang, Shinya Kasai,Seiji Mitani, Koichiro Inomata, and Kazuhiro Hono, APPLIED PHYSICSLETTERS 96, 212505 [1] (2010)

[Non Patent Document 2] Thomas Scheike, Hiroaki Sukegawa, TakaoFurubayashi, Zhenchao Wen, Koichiro Inomata, Tadakatsu Ohkubo, KazuhiroHono, and Seiji Mitani, Applied Physics Letters, 105, 242407 (2014)

[Non Patent Document 3] Yoshio Miura, Shingo Muramoto, Kazutaka Abe, andMasafumi Shirai, Physical Review B 86, 024426 (2012)

DISCLOSURE OF INVENTION Problem to be Solved by Invention

Information writing with spin-transfer torque (STT) has been brought toattention in the field of MRAM. The information to be written can bedetermined based on a direction of current by applying the currentdensity by which the information is writable to a magnetic element.However, since the current for rewriting the information generallyvaries in accordance with the direction of the current which is appliedto the magnetoresistance effect element, there is a problem thateasy-to-rewrite level also varies in accordance with the information.

One of the factors causing the problem, i.e. the differenteasy-to-rewrite level of the information, is a MR ratio which variesaccording to a direction of current flowing the magnetoresistance effectelement. In a case of STT, the easy-to-rewrite level is proportional toamplitude of the MR ratio. That is, the amount of current for rewritingthe information varies as the MR ratio varies in accordance with thedirection of the current. Therefore, the magnetoresistance effectelement independent from the direction of current bias (i.e. withexcellent symmetry for the current bias) is required as a MRAM or aswitch. Moreover, in a case where the magnetoresistance effect elementis used as a high-frequency oscillator or a wave detector, themagnetoresistance effect element independent from the direction ofcurrent bias is also required. The frequency stability is improved asthe similarity for the current bias is enhanced in high-frequencyapplications.

In Patent Document 1 and Non Patent Document 1, a tunnel barrier havinga spinel structure is reported to be effective as a substituent materialfor MgO. A spinel tunnel barrier expressed by a composition formula ofMgAl₂O₄ has been known to obtain the same MgO ratio as MgO, and toobtain a higher MR ratio than MgO at a high bias voltage. In addition,in Patent Document 2 and Non Patent Documents 2 and 3, there is adescription that MgAl₂O₄ is required to have a disordered spinelstructure in order to obtain a high MR ratio. The disordered spinelstructure mentioned herein refers to a cubic structure as a whole, inwhich the arrangement of O atoms has almost the same close-packed cubiclattice as a spinel, but the arrangements of Mg atoms and Al atoms aredisordered. In an original spinel, Mg and Al are arranged withregularity in tetrahedral voids and octahedral voids of oxygen ions.However, since these are randomly arranged in a disordered spinelstructure, the symmetry of crystal changes, and thus a structure inwhich the lattice constant is substantially reduced by half from about0.808 nm of MgAl₂O₄ is obtained.

An object of the invention is to provide a magnetoresistance effectelement which is able to effectively carry out the magnetizationreversal due to the current and has more excellent symmetry of the MRratio for the bias voltage compared with the TMR element using MgO, aconventional material of a tunnel barrier layer.

Means for Solving the Problems

In order to solve the above-described problems, a magnetoresistanceeffect element according to the invention has a first ferromagneticmetal layer, a second ferromagnetic metal layer, and a tunnel barrierlayer that is sandwiched between the first and second ferromagneticmetal layers, and the tunnel barrier layer has a spinel structurerepresented by a composition formula AGa₂O_(x) (0<x≤4), in which A is anon-magnetic divalent cation which is one or more selected from a groupconsisting of magnesium, zinc and cadmium.

The spinel material containing potassium and oxygen exhibits a symmetricMR ratio regardless of the sign of the bias voltage, thus spinpolarizablity is difficult to be attenuated at a region of low bias.Accordingly, the information is rewritable at the lower voltage thanthat of the conventional material.

In the magnetoresistance effect element, the tunnel barrier layer mayhave a lattice-matched portion that is lattice-matched with both of thefirst ferromagnetic metal layer and the second ferromagnetic metallayer, and a lattice-mismatched portion that is not lattice-matched withat least one of the first ferromagnetic metal layer and the secondferromagnetic metal layer.

In the magnetoresistance effect element, a volume ratio of thelattice-matched portion in the tunnel barrier layer with respect to avolume of the entire tunnel barrier layer may be 65% to 95%.

In a case where the volume ratio of the lattice-matched portion in thetunnel barrier layer is 65% or less, the effect of coherent tunneling isreduced, and thus the MR ratio decreases. In a case where the volumeratio of the lattice-matched portion in the tunnel barrier layer is 95%or greater, the interference effect between the spin-polarized electronsduring passing through the tunnel barrier layer does not decrease, andthus an increase in passage of the spin-polarized electrons through thetunnel barrier layer is not observed. By making the number ofconstituent elements of the non-magnetic element smaller than half thenumber of elements of the aluminum ion, vacancies are generated in thecation sites, the vacancies and two or more types of non-magneticelements occupy the cations, and thus lattice periodicity is disturbed.Accordingly, the MR ratio is further increased.

In the magnetoresistance effect element, the tunnel barrier layer mayhave a disordered spinel structure. In a case where the tunnel barrierlayer is disordered, the MR ratio is increased.

In the magnetoresistance effect element, a crystal lattice of either orboth of the first ferromagnetic metal layer and the second ferromagneticmetal layer may be matched with a crystal lattice of the tunnel barrierlayer to form a cubic-on-cubic structure. The crystal lattice of thespinel material containing potassium and oxygen is matched with thecrystal lattice of the ferromagnetic material such as iron, therebyforming the cubic-on-cubic structure. Therefore, since scattering at aninterface between the tunnel barrier layer and the ferromagnetic metallayer is suppressed, bias dependency of the MR ratio exhibits asymmetric structure regardless of the direction of voltage. Accordingly,the information is rewritable at the lower voltage than that of theconventional material.

In the magnetoresistance effect element, a composition ratio of thedivalent cation and the trivalent cation (Ga ion) may be within a rangeof 0.9 to 1.25:2. In a case where the composition ratio of the divalentcation and the trivalent cation is approximately 1:2, the structure isdifficult to be disordered and thus bias symmetry of the MR ratio isimproved. Accordingly, the information is rewritable at the lowercurrent than that of the conventional material.

In the magnetoresistance effect element, the first ferromagnetic metallayer may have larger coercivity than the second ferromagnetic metallayer. Since the coercivity of the first ferromagnetic metal layer isdifferent from that of the second ferromagnetic metal layer, the elementfunctions as a spin valve, and device application is possible.

In the magnetoresistance effect element, a magnetoresistance ratio maybe 100% or greater when applying the voltage of 1 V or higher at roomtemperature. Accordingly, the magnetoresistance effect element isavailable in devices to which the high bias voltage is applied, e.g. ahigh-sensitive magnetic sensor, a logic-in-memory and a MRAM.

In the magnetoresistance effect element, at least one of the firstferromagnetic metal layer and the second ferromagnetic metal layer mayhave magnetic anisotropy perpendicular to a lamination direction. Sinceit is not necessary to apply a bias magnetic field, it is possible toreduce the device in size. In addition, the element can be allowed tofunction as a recording element since it has high thermal disturbanceresistance.

In the magnetoresistance effect element, at least one of the firstferromagnetic metal layer and the second ferromagnetic metal layer maybe Co₂Mn_(1-a)Fe_(a)Al_(b)Si_(1-b) (0≤a≤1, 0≤b≤1).Co₂Mn_(1-a)Fe_(a)Al_(b)Si_(1-b) is a ferromagnetic metal material havinghigh spin polarizability, and a higher MR ratio can be obtained than ina case where another ferromagnetic metal material is used.

Effects of the Invention

According to the invention, it is possible to provide amagnetoresistance effect element which is able to effectively carry outthe magnetization reversal due to the current and has more excellentsymmetry of the MR ratio for the bias voltage compared with the TMRelement using MgO or MgAl₂O₄, conventional materials of the tunnelbarrier layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a lamination structure of a magnetoresistance effectelement.

FIG. 2 is a diagram of a crystal structure of a spinel.

FIG. 3 is a schematic diagram of an ordered spinel structure and adisordered spinel structure with symmetry of Fm-3m having a latticeconstant assumed in a tunnel barrier layer of the invention.

FIG. 4 is a schematic diagram of an ordered spinel structure and adisordered spinel structure with symmetry of Fm-3m having a latticeconstant assumed in a tunnel barrier layer of the invention.

FIG. 5 is a schematic diagram of an ordered spinel structure and adisordered spinel structure with symmetry of Fm-3m having a latticeconstant assumed in a tunnel barrier layer of the invention.

FIG. 6 is a schematic diagram of an ordered spinel structure and adisordered spinel structure with symmetry of F-43m having a latticeconstant assumed in a tunnel barrier layer of the invention.

FIG. 7 is a schematic diagram of an ordered spinel structure and adisordered spinel structure with symmetry of F-43m having a latticeconstant assumed in a tunnel barrier layer of the invention.

FIG. 8 is a diagram showing an evaluation structure of amagnetoresistance effect element according to an embodiment in adirection perpendicular to a lamination direction.

FIG. 9 is a diagram showing the element structure according to theembodiment in the lamination direction.

FIG. 10 is a diagram showing results of evaluation of amagnetoresistance effect of a magnetoresistance effect element ofExample 1.

FIG. 11 is a diagram showing a decreasing rate of the MR ratio at thebias voltage on the basis of the MR ratio at the low bias voltage ofExample 1 as a reference.

FIG. 12 is a diagram showing results of evaluation of amagnetoresistance effect of a magnetoresistance effect element ofExample 2.

FIG. 13 is a diagram showing a decreasing rate of the MR ratio at thebias voltage on the basis of the MR ratio at the low bias voltage ofExample 2 as a reference.

FIG. 14 is a diagram showing results of evaluation of amagnetoresistance effect of a magnetoresistance effect element ofExample 3.

FIG. 15 is a diagram showing a decreasing rate of the MR ratio at thebias voltage on the basis of the MR ratio at the low bias voltage ofExample 3 as a reference.

FIG. 16 is a diagram showing results of evaluation of amagnetoresistance effect of a magnetoresistance effect element ofExample 4.

FIG. 17 is a diagram showing a decreasing rate of the MR ratio at thebias voltage on the basis of the MR ratio at the low bias voltage ofExample 4 as a reference.

FIG. 18 is a diagram showing a maximum value of a reversal current and aconcentration ratio of Mg to Ga in a case where a concentration of Ga is2 in Example 5.

FIG. 19 is a diagram showing results of evaluation of amagnetoresistance effect of a magnetoresistance effect element ofComparative Example 1.

FIG. 20 is a diagram showing a decreasing rate of the MR ratio at thebias voltage on the basis of the MR ratio at the low bias voltage ofComparative Example 1 as a reference.

FIG. 21 shows an example of a part in which the tunnel barrier layer andthe ferromagnetic metal layer are lattice-matched. FIG. 21(a) shows ahigh-resolution cross-section TEM image. FIG. 21(b) shows an example ofan image obtained by performing inverse Fourier analysis.

FIG. 22 is a diagram showing a structure of a cross-section including adirection parallel to a lamination direction of Example 8.

FIG. 23 is a diagram showing a proportion of a lattice-matched portionin which a tunnel barrier layer of Example 9 is lattice-matched withboth of a first ferromagnetic metal layer and a second ferromagneticmetal layer, and characteristics of an element. FIG. 23(A) is a diagramshowing element resistance (Rp) when magnetization directions of thefirst ferromagnetic metal layer and the second ferromagnetic metal layerare parallel to each other. FIG. 23(B) is a diagram showing elementresistance (Rap) when magnetization directions of the firstferromagnetic metal layer and the second ferromagnetic metal layer areantiparallel to each other. FIG. 23(C) is a diagram showing amagnetoresistance ratio of the element.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the invention will be described in detailwith reference to the accompanying drawings. In the description of thedrawings, the same elements will be denoted by the same referencenumerals, and an overlapping description thereof will be omitted.

First Embodiment

Hereinafter, a case where a magnetoresistance effect element accordingto a first embodiment has a first ferromagnetic metal layer 6, a secondferromagnetic metal layer 7, and a tunnel barrier layer 3 sandwichedbetween the first and second ferromagnetic metal layers, and the tunnelbarrier layer 3 has a spinel structure represented by a compositionformula AGa₂O_(x) (0<x≤4), and A is a non-magnetic divalent cation whichis one or more selected from a group consisting of magnesium, zinc andcadmium, will be described.

(Basic Structure)

In the example shown in FIG. 1, a magnetoresistance effect element 100is provided on a substrate 1, and has a stacked structure provided withan underlayer 2, a first ferromagnetic metal layer 6, a tunnel barrierlayer 3, a second ferromagnetic metal layer 7, and a cap layer 4 inorder from the substrate 1.

(Tunnel Barrier Layer)

The tunnel barrier layer 3 is made of a non-magnetic insulatingmaterial. In general, the tunnel barrier layer has a film thickness of 3nm or less, and in a case where the tunnel barrier layer is sandwichedbetween metal materials, a wave function of electrons of atoms of themetal materials extends beyond the tunnel barrier layer 3, and thus acurrent may flow regardless of the presence of an insulating material onthe circuit. The magnetoresistance effect element 100 is classified intotwo types including: a type in which the typical tunneling effect isused; and a type in which the coherent tunneling effect where an orbitfor tunneling is limited is predominant. In the typical tunnelingeffect, a magnetoresistance effect is obtained by spin polarization offerromagnetic materials. On the other hand, in the coherent tunneling,an orbit for tunneling is limited. Therefore, in a magnetoresistanceeffect element in which coherent tunneling is predominant, an effecthigher than or equivalent to spin polarization of ferromagnetic metalmaterials can be expected. In order to exhibit the coherent tunnelingeffect, it is necessary that the ferromagnetic metal materials and thetunnel barrier layer 3 be crystallized and joined in a specificorientation.

(Spinel Structure)

FIG. 2 shows a crystal structure of a spinel. An A-site in which oxygenis fourfold coordinated to cations and a B-site in which oxygen issixfold coordinated to cations exist. Here, a Sukenel structurereferring to the spinel structure in which cations are disordered is astructure that has a lattice constant half the lattice constant of anordered spinel structure while a position of an oxygen atom of theordered spinel is almost not changed, and in which cations arepositioned in tetrahedral positions and octahedral positions of oxygenatoms that are not occupied under ordinary circumstances. At this time,this structure may include total five structures shown in FIGS. 3 to 7,and may be any one of them or a mixed structure thereof.

(Definition of Disordered Spinel Structure)

In this specification, the spinel structure in which cations aredisordered may be referred to as a Sukenel structure. The Sukenelstructure refers to a structure where oxygen atoms are arranged in cubicclose-packed lattice that is substantially similar to spinel lattice,the structure as a whole belongs to a cubic structure, but arrangementof cations are disordered. In an original ordered spinel, Mg and Al arearranged in order in the tetrahedral vacancies and octahedral vacanciesin the original spinel. However, since these are arranged in randomarrangement in the Sukenel structure, the crystal symmetry of thestructure is different from MgAl₂O₄, and the lattice constant of thestructure is substantially half of that of MgAl₂O₄. With a change in thelattice-repeating unit, a combination between the ferromagnetic layermaterial and the electronic structure (band structure) is changed, andthus a large TMR enhancement due to a coherent tunneling effect isobtained. For example, a space group of MgAl₂O₄ that is a non-magneticspinel material is Fd-3m, but a space group of a disordered spinelstructure with a lattice constant reduced by half is known to be changedto Fm-3m or F-43m, and there are a total of five structures (Non-PatentDocument 2). Any one of them can be used.

In this specification, the Sukenel structure is not essentially requiredto be a cubic structure. In the stacked structure, the crystal structureis influenced by the crystal structure of the material of an underlayer,and the lattice is thus partially distorted. Each material has a bulkcrystal structure, but in a case where it is formed into a thin film, apartially distorted crystal structure based on the bulk crystalstructure can be taken. Particularly, in the invention, the tunnelbarrier layer has a very thin structure, and is easily influenced by thecrystal structure of the layer brought into contact with the tunnelbarrier layer. In this regard, the bulk crystal structure of a Sukenelstructure is a cubic structure, and in this specification, the Sukenelstructure includes a Sukenel structure which does not have a cubicstructure in addition to a Sukenel structure slightly deviating from thecubic structure. A deviation from the cubic structure in the Sukenelstructure described in this specification is generally slight, and thisdeviation depends on the accuracy of a measurement method for evaluatingthe structure.

The divalent cation (A-site) in the non-magnetic element of the tunnelbarrier layer is one or more selected from the group consisting ofmagnesium, zinc, and cadmium. These non-magnetic elements are stable ina case where the number of valence is 2, and in a case where thesenon-magnetic elements are constituent elements of the tunnel barrierlayer, coherent tunneling can be realized, and the MR ratio isincreased.

In a case where the divalent cation (A-site) contained in the tunnelbarrier layer includes a plurality of non-magnetic elements, adifference in ionic radius between the divalent cations of the pluralityof non-magnetic elements is preferably 0.2 A or less. In a case wherethe difference in ionic radius is small, the cations are unlikely to beordered, and thus the lattice constant becomes smaller than that of ageneral spinel structure. Accordingly, the MR ratio is further increasedin a case of two or more types of elements that are similar to eachother in ionic radius.

(First Ferromagnetic Metal Layer)

Examples of the material of the first ferromagnetic metal layer 6include metal selected from the group consisting of Cr, Mn, Co, Fe, andNi, alloy including one or more of the metals of the group, and alloyincluding one or more metals selected from the group and at least oneelement of B, C, and N. Specific examples thereof include Co—Fe andCo—Fe—B. A Heusler alloy such as Co₂FeSi is preferable in order toobtain a high output. The Heusler alloy includes intermetallic compoundshaving a chemical composition of X₂YZ. X denotes a Co, Fe, Ni, or Cugroup transition metal element or noble metal in the periodic table, Ydenotes a Mn, V, Cr, or Ti group transition metal, and can also take theelemental species of X, and Z denotes representative elements of III toV groups. Examples thereof include Co₂FeSi, Co₂MnSi, andCo₂Mn_(1-a)Fe_(a)Al_(b)Si_(1-b). In addition, an antiferromagneticmaterial such as IrMn and PtMn may be used as a material brought intocontact with the first ferromagnetic metal layer 6 in order to makecoercivity of the first ferromagnetic metal layer larger than that ofthe second ferromagnetic metal layer 7. Furthermore, the firstferromagnetic metal layer may have a synthetic ferromagnetic couplingstructure such that the second ferromagnetic metal layer 7 is notinfluenced by a leakage magnetic field of the first ferromagnetic metallayer 6.

In a case where a magnetization direction of the first ferromagneticmetal layer 6 is made perpendicular to the stacked plane, a stacked filmof Co and Pt is preferably used. For example, in a case where the firstferromagnetic metal layer 6 has a composition of [Co (0.24 nm)/Pt (0.16nm)]₆/Ru (0.9 nm)/[Pt (0.16 nm)/Co (0.16 nm)]₄/Ta (0.2 nm)/FeB (1.0 nm),the magnetization direction can be made perpendicular to the stackedplane.

(Second Ferromagnetic Metal Layer)

A ferromagnetic material, particularly, a soft magnetic material isapplied as a material of the second ferromagnetic metal layer 7, andexamples thereof include metal selected from the group consisting of Cr,Mn, Co, Fe, and Ni, alloy including one or more of the metals of thegroup, and alloy including one or more metals selected from the groupand at least one element of B, C, and N. Specific examples thereofinclude Co—Fe, Co—Fe—B, and Ni—Fe.

In a case where a magnetization direction of the second ferromagneticmetal layer 7 is made perpendicular to the stacked plane, the secondferromagnetic metal layer 7 preferably has a thickness of 2.5 nm orless. Perpendicular magnetic anisotropy can be applied to the secondferromagnetic metal layer 7 at an interface between the secondferromagnetic metal layer 7 and the tunnel barrier layer 3. The secondferromagnetic metal layer 7 preferably has a thin film thickness sincethe effect of the perpendicular magnetic anisotropy is reduced if thesecond ferromagnetic metal layer 7 has a thick film thickness.

In general, the first ferromagnetic metal layer 6 has a structure inwhich the magnetization direction thereof is fixed, and is called afixed layer. In addition, since the second ferromagnetic metal layer 7has a magnetization direction that can be more easily changed by anexternal magnetic field or a spin torque than the first ferromagneticmetal layer 6, the second ferromagnetic metal layer is called a freelayer.

(Substrate)

A magnetoresistance effect element according to the invention may beformed on a substrate.

In that case, a material showing excellent flatness is preferably usedas a material of the substrate 1. The substrate 1 differs depending onthe purpose. For example, in a case of MRAM, a circuit formed in a Sisubstrate can be used under the magnetoresistance effect element. In acase of a magnetic head, an AlTiC substrate that can be easily processedcan be used.

(Underlayer)

In a case where a magnetoresistance effect element according to theinvention is formed on a substrate, first, an underlayer may be formedon the substrate.

In that case, the underlayer 2 is used to control crystallinity such ascrystal orientation and crystal grain size of the first ferromagneticmetal layer 6 and layers formed above the first ferromagnetic metallayer 6. Therefore, it is important to select the material of theunderlayer 2. Hereinafter, the material and the configuration of theunderlayer 2 will be described. Any of a conductive material and aninsulating material may be used for the underlayer, but in a case whereelectric power is fed to the underlayer, a conductive material ispreferably used. First, as a first example of the underlayer 2, anitride layer having a (001)-oriented NaCl structure and containing atleast one element selected from the group consisting of Ti, Zr, Nb, V,Hf, Ta, Mo, W, B, Al, and Ce is exemplified. As a second example of theunderlayer 2, a (002)-oriented perovskite conductive oxide layer made ofRTO₃ is exemplified. Here, the R-site includes at least one elementselected from the group consisting of Sr, Ce, Dy, La, K, Ca, Na, Pb, andBa, and the T-site includes at least one element selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta, Ce, andPb. As a third example of the underlayer 2, an oxide layer having a(001)-oriented NaCl structure and containing at least one elementselected from the group consisting of Mg, Al, and Ce is exemplified. Asa fourth example of the underlayer 2, a layer having a (001)-orientedtetragonal or cubic structure and containing at least one elementselected from the group consisting of Al, Cr, Fe, Co, Rh, Pd, Ag, Ir,Pt, Au, Mo, and W is exemplified. As a fifth example of the underlayer2, a layer having a stacked structure with a combination of two or moreof the layers of the above first to fourth examples is exemplified. Bydevising the structure of the underlayer as described above, it ispossible to control the crystallinity of the ferromagnetic layer 2 andlayers formed above the ferromagnetic layer 2, thereby improving themagnetic characteristics.

(Cap Layer)

A cap layer may be formed on the second ferromagnetic metal layer of themagnetoresistance effect element according to the invention.

A cap layer 4 is installed above the second ferromagnetic metal layer 7in a stacking direction in order to control crystallinity such ascrystal orientation and crystal grain size and element diffusion. In acase where a free layer having a bcc structure is formed, the crystalstructure of the cap layer may be any one of a fcc structure, a hcpstructure, and a bcc structure. In a case where a free layer having afcc structure is formed, the crystal structure of the cap layer may beany one of a fcc structure, a hcp structure, and a bcc structure. Thefilm thickness of the cap layer may be within such a range that adistortion relaxation effect is obtained and a reduction in the MR ratioby shunt is not shown. The film thickness of the cap layer is preferably1 nm to 30 nm.

(Shape and Dimensions of Element)

A laminate formed of the first ferromagnetic metal layer, the tunnelbarrier layer, and the second ferromagnetic metal layer 2 constitutingthe invention has a columnar shape. In addition, it may have variousshapes such as a circular shape, a square shape, a triangle shape, and apolygonal shape when viewed from top, and preferably has a circularshape from the viewpoint of symmetry. That is, the laminate preferablyhas a columnar shape.

FIGS. 8 and 9 show examples of the shape and the dimensions of themagnetoresistance effect element.

FIG. 8 is a diagram showing a structure when viewed from a side in astacking direction of the magnetoresistance effect element 100. Themagnetoresistance effect element 100 of FIG. 8 has an electrode layer 5formed above the cap layer 4 shown in FIG. 1. FIG. 9 is a diagramshowing a structure when viewed in the stacking direction of themagnetoresistance effect element 100. In FIG. 9, a current source 71 anda voltmeter 72 are also shown.

The magnetoresistance effect element 100 is processed into a columnarshape of 80 nm or less as shown in FIGS. 8 and 9, and wiring is applied.Since the magnetoresistance effect element 100 is processed into acolumnar shape having a size of 80 nm or less, a domain structure is notlikely to be formed in the ferromagnetic metal layers, and it is notnecessary to consider a component having a different spin polarizationin the ferromagnetic metal layers In FIG. 9, the magnetoresistanceeffect element 100 is disposed at a position where the underlayer 2 andthe electrode layer 5 intersect each other.

(Evaluation Method)

The magnetoresistance effect element 100 can be evaluated with thestructure shown in FIGS. 8 and 9. For example, the power supply 71 andthe voltmeter 72 are disposed as shown in FIG. 9 such that a fixedcurrent or a fixed voltage is applied to the magnetoresistance effectelement 100. By measuring the voltage or the current while sweeping anexternal magnetic field, a change in the resistance of themagnetoresistance effect element 100 can be measured.

In general, the MR ratio is expressed by the following formula.MR Ratio(%)={(R _(AP) −R _(P))/R _(P)}×100

R_(P) denotes a resistance in a case where magnetization directions ofthe first ferromagnetic metal layer 6 and the second ferromagnetic metal7 are parallel to each other, and R denotes a resistance in a case wheremagnetization directions of the first ferromagnetic metal layer 6 andthe second ferromagnetic metal 7 are antiparallel to each other.

In a case where a strong current flows in the magnetoresistance effectelement 100, magnetization rotation occurs by a STT effect, and aresistance value of the magnetoresistance effect element 100 is rapidlychanged. The current value at which the resistance value is rapidlychanged is called a reversal current value (Jc).

(Others)

In this embodiment, the structure has been exemplified in which thefirst ferromagnetic metal layer 6 having high coercivity is disposed onthe lower side, but the invention is not limited to this structure. In acase of a structure in which the first ferromagnetic metal layer 6having high coercivity is disposed on the upper side, the coercivity isreduced in comparison with a case in which the first ferromagnetic metallayer 6 is disposed on the lower side, but the tunnel barrier layer 3can be formed by utilizing the crystallinity of the substrate, and thusthe MR ratio can be increased.

In order to utilize the magnetoresistance effect element as a magneticsensor, a resistance change preferably changes linearly with respect toan external magnetic field. In a general laminated film of ferromagneticlayers, magnetization directions are easily directed into the laminationplane by shape anisotropy. In this case, for example, a magnetic fieldis applied from outside to make the magnetization directions of thefirst ferromagnetic metal layer and the second ferromagnetic metal layerintersect each other, thereby changing the resistance change linearlywith respect to the external magnetic field. However, in this case,since a mechanism that applies a magnetic field is required near themagnetoresistance effect element, this is not preferable forintegration. In a case where the ferromagnetic metal layer itself hasperpendicular magnetic anisotropy, this is advantageous for integrationsince a method such as application of a magnetic field from outside isnot required.

The magnetoresistance effect element using this embodiment can be usedas a magnetic sensor or a memory such as a MRAM. Particularly, thisembodiment is effective for products that are used with a bias voltagehigher than a bias voltage used in conventional magnetic sensors.

(Manufacturing Method)

The magnetoresistance effect element 100 can be formed using, forexample, a magnetron sputtering apparatus.

The tunnel barrier layer 3 can be produced through a known method. Forexample, a thin metal film is formed on the first ferromagnetic metallayer 6 by sputtering, performing plasma oxidation or natural oxidationby oxygen introduction thereon, and performing a heat treatment thereon.As the film-forming method, not only a magnetron sputtering method butalso a thin film-forming method such as a vapor deposition method, alaser ablation method, or a MBE method can be used.

Each of the underlayer, the first ferromagnetic metal layer, the secondferromagnetic metal layer, and the cap layer can be formed through aknown method.

Second Embodiment

A second embodiment is different from the first embodiment only in themethod of forming a tunnel barrier layer. In the first embodiment, thetunnel barrier layer is formed by repeatedly performing formation andoxidation of a metal film. In the second embodiment, the substratetemperature is lowered to −70 to −30 degrees, and then oxidation isperformed in the oxidation step. By cooling the substrate, a temperaturegradient is generated between the substrate and the vacuum or betweenthe substrate and the plasma. First, in a case where a surface of thesubstrate is exposed to oxygen, oxygen reacts with the metal materialand the metal material is oxidized. However, the oxidation does notproceed due to the low temperature. Accordingly, the oxygen amount ofthe tunnel barrier layer is easily adjusted. Moreover, by forming thetemperature gradient, epitaxial growth (lattice-matched growth) iseasily adjusted. Since the crystal growth proceeds by the temperaturegradient, the epitaxial growth is easily performed in a case where thetemperature of the substrate is sufficiently lowered. As the temperatureof the substrate is increased, domains are formed and a plurality ofcrystal nuclei are thus formed in the plane. Each of the crystal nucleiis independently and epitaxially grown, and thus a part in whichlattices are not matched is formed in a part in which the grown domainsare in contact with each other.

It is preferable that in the tunnel barrier layer, lattice-matched;parts, which are lattice-matched with both of a first ferromagneticmetal layer and a second ferromagnetic metal layer, partially exist. Ingeneral, it is preferable that the tunnel barrier layer be completelylattice-matched to both of the first ferromagnetic metal layer and thesecond ferromagnetic metal layer. However, in a case where the tunnelbarrier layer is completely lattice-matched, spin-polarized electronsinterfere with each other during passing through the tunnel barrierlayer, and thus the electrons do not easily pass through the tunnelbarrier layer. In contrast, in a case where lattice-matched parts, inwhich lattices are matched, partially exist, the interference betweenspin-polarized electrons during passing through the tunnel barrier layeris appropriately cut in parts in which lattices are not matched, andthus the spin-polarized electrons easily pass through the tunnel barrierlayer. The volume ratio of the lattice-matched part portion in thetunnel barrier layer with respect to the volume of the entire tunnelbarrier layer is preferably 65% to 95%. In a case where the volume ratioof the lattice-matched part in the tunnel barrier layer is 65% or less,the effect of coherent tunneling is reduced, and thus the MR ratiodecreases. In a case where the volume ratio of the lattice-matched partin the tunnel barrier layer is 95% or greater, the interference effectbetween the spin-polarized electrons during passing through the tunnelbarrier layer is not be weakened, and thus an increase in passage of thespin-polarized electrons through the tunnel barrier layer is notobserved.

(Method of Calculating Volume Ratio of Lattice-Matched Portion)

The volume ratio of the lattice-matched part (lattice-matched portion)with respect to the volume of the entire tunnel barrier layer can beestimated from, for example, a TEM image. Regarding whether the latticesare matched, a part including the tunnel barrier layer, the firstferromagnetic metal layer, and the second ferromagnetic metal layer in across-section TEM image is Fourier-transformed to obtain an electronbeam diffraction image. In the electron beam diffraction image obtainedby Fourier transformation, electron beam diffraction spots in directionsother than the stacking direction are removed. That image is subjectedto inverse Fourier transformation to provide an image in whichinformation only in the stacking direction is obtained. In lattice linesin the inverse Fourier image, a part in which the tunnel barrier layeris continuously connected to both of the first ferromagnetic metal layerand the second ferromagnetic metal layer is defined as a lattice-matchedportion. In addition, in lattice lines, a part in which the tunnelbarrier layer is not continuously connected to at least one of the firstferromagnetic metal layer and the second ferromagnetic metal layer, orin which no lattice lines are detected, is defined as alattice-mismatched portion. In the lattice-matched portion, in thelattice lines in the inverse Fourier image, the layers are continuouslyconnected from the first ferromagnetic metal layer to the secondferromagnetic metal layer via the tunnel barrier layer, and thus a width(L_(C)) of the lattice-matched portion can be measured from the TEMimage. Similarly, in the lattice-mismatched portion, in the latticelines in the inverse Fourier image, the layers are not continuouslyconnected, and thus a width (L_(I)) of the lattice-mismatched portioncan be measured from the TEM image. Using the width (L_(C)) of thelattice-matched portion as a numerator and using the sum of the width(L_(C)) of the lattice-matched portion and the width (L_(I)) of thelattice-mismatched portion as a denominator, the volume ratio of thelattice-matched portion with respect to the volume of the entire tunnelbarrier layer can be obtained. The TEM image is a cross-section image,but includes information including a depth. Accordingly, it can bethought that the region estimated from the TEM image is proportional tothe volume.

FIG. 21 shows an example of the part in which the tunnel barrier layerand the ferromagnetic metal layer are lattice-matched. FIG. 21 (A) showsan example of a high-resolution cross-section TEM image. FIG. 21 (B)shows an example of an image obtained by performing inverse Fouriertransformation after removal of electron beam diffraction spots indirections other than the stacking direction in the electron beamdiffraction image. In FIG. 21 (B), components perpendicular to thestacking direction are removed, and thus lattice lines can be observedin the stacking direction. This shows that the tunnel barrier layer andthe ferromagnetic metal layer are continuously connected to each otherwithout interruption at an interface therebetween.

EXAMPLES Example 1

Hereinafter, an example of the method of manufacturing amagnetoresistance effect element according to the first embodiment willbe described. Film formation was performed on a substrate provided witha thermal silicon oxide film using a magnetron sputtering method. As anunderlayer, 5 nm of Ta/3 nm of Ru was formed, and as a firstferromagnetic metal layer, [Co (0.24 nm)/Pt (0.16 nm)]₆/Ru (0.9 nm)/[Pt(0.16 nm)/Co (0.16 nm)]₄/Ta (0.2 nm)/FeB (1.0 nm) was formed in orderthereon. Next, a method of forming a tunnel barrier layer will be shown.A 0.5 nm thick film of MgGa₂ is formed by sputtering with a targethaving an alloy composition of MgGa₂. Thereafter, the above-describedsample was moved to an oxidation chamber of which the inside was kept inan ultrahigh vacuum of 1×10⁻⁸ Pa or less to perform natural oxidation byintroducing Ar and oxygen. The natural oxidation time was 10 seconds,the partial pressure ratio of Ar to oxygen was 1 to 25, and the totalgas pressure was 0.05 Pa. Then, the sample was returned to a filmforming chamber, and a film of MgGa₂ with a thickness of 0.4 m wasformed. The above-described sample was moved to the oxidation chamber ofwhich the inside was kept in an ultrahigh vacuum of 1×10⁻⁸ Pa or less toperform natural oxidation and inductively coupled plasma oxidation byintroducing Ar and oxygen. The natural oxidation time was 30 seconds,and the inductively coupled plasma oxidation time was 5 seconds. Thepartial pressure ratio of Ar to oxygen was 1 to 20, and the total gaspressure was 0.08 Pa.

The above-described stacked film was moved again to the film formingchamber, and a 2 nm thick FeB film was formed as a second ferromagneticmetal layer 7. 3 nm of Ru/5 nm of Ta was formed as a cap layer 4.

The above-described stacked film was installed in an annealingapparatus, and treated for 10 minutes at a temperature of 450° C. in Ar.Then, in a state in which a magnetic field of 8 kOe was applied, thestacked film was heated at a temperature of 280° C. for 6 hours.

Next, an element was formed as in FIG. 9. First, a photoresist wasformed using electron beam lithography in such a way that the electrodelayer was in a direction rotated by 90 degrees as in FIG. 9. A partother than a part below the photoresist was eliminated by an ion millingmethod to expose the thermal silicon oxide film that was the substrate,and thus a shape of the underlayer 2 was formed. In a narrow part in theshape of the underlayer, a photoresist was formed into a cylindricalshape of 80 nm using electron beam lithography, and a part other than apart below the photoresist was eliminated by an ion milling method toexpose the underlayer. Thereater, SiOx was formed as an insulating layeron the part shaved by ion milling. Here, the photoresist with acylindrical shape of 80 nm was removed. The photoresist was not formedonly in a part corresponding to an electrode pad of FIG. 9, and theinsulating layer was removed by an ion milling method to expose theunderlayer. Thereafter, an Au layer was formed. This electrode pad 8functions as a contact electrode for the underlayer of theabove-described stacked film. Next, a photoresist was formed and shapingwas performed by an ion milling method such that the electrode layer ofFIG. 9 was formed, and an Au film was formed. This functions as acontact electrode for the electrode layer of the above-described stackedfilm.

(Characteristics of Example 1)

The magnetoresistance effect element evaluation method is based on amagnetoresistance effect element evaluation method that has beengenerally performed. As shown in FIG. 9, a current source and avoltmeter were connected to the electrode pad and the electrode layer,respectively, to perform measurement by a four-terminal method. FIG. 10is a diagram showing results of evaluation of the magnetoresistanceeffect of the magnetoresistance effect element of Example 1. Thehorizontal axis represents an applied current, and the vertical axisrepresents the resistance of the element. The current was applied from acurrent source to the magnetoresistance effect element to measure thevoltage. In FIG. 10, resistance values were calculated and plotted interms of a correlation between the current and the voltage. The currentflowing from the second ferromagnetic metal layer to the firstferromagnetic metal layer was determined as a current flowing in thepositive direction, while the current flowing reversely was determinedas a current flowing in the negative direction. Moreover, the evaluationwas performed in a state where the element resistance was rapidlychanged by increasing the current and thus the current gradually changedto a current opposite to the direction thereof and reducing the currentby a current value in which magnetization directions of the first andsecond ferromagnetic metal layers were varied from parallel toantiparallel or vice versa. The direction of an arrow shown in FIG. 10indicates the order in which the element resistance was changed. FromFIG. 10, it was recognized that the resistance was rapidly increasedwith the current of 1 mA and rapidly decreased with the current of −1.05mA. Therefore, the reversal current values are 1 mA and −1.05 mA,respectively. Based on these results, it was found that themagnetization directions of the first and second ferromagnetic metallayers were gradually changed due to the STT effect occurred by thecurrent and thus the element resistance was changed. It was also foundthat the area resistance (RA) was 0.6 Ω·μm² in a case where themagnetization directions of the first and second ferromagnetic metallayers were parallel to each other. The MR ratio was 64.8% in a casewhere the bias voltage was 1 V. This tunnel barrier layer was confirmedas a spinel structure consisting of a cubic structure from an electronbeam diffraction image.

The MR ratio was calculated using the results shown in FIG. 10. FIG. 11is a diagram showing a decreasing rate of the MR ratio at the biasvoltage on the basis of the MR ratio at the low bias voltage as areference. In the drawing, the horizontal axis represents an appliedcurrent, and the vertical axis represents a decreasing rate of the MRratio on the basis of the MR ratio in a case where the bias voltage of0.001 V was applied. From FIG. 11, it was found that the voltage(V_(half)) at which the MR ratio decreased by half was 1.05 V or −1 V.V_(half) is one of the indices indicating decrease in the MR ratio underthe bias voltage. V_(half) represents a bias voltage at which the MRratio decreases by half compared with the MR ratio at the time when thelow bias voltage was applied on the basis of the low bias voltage as areference. The low bias voltage is, for example, 1 mV. Furthermore,since the optimal low bias voltage varies according to the conditionssuch as the resistance value of the magnetoresistance effect element, itis needed that the low bias voltage is at least a voltage equal to orless than a tenth part of V_(half).

Example 2

The production method is similar to that in the Example, but only thematerial for forming the tunnel barrier layer is different from that ofExample. A 0.5 nm thick film of ZnGa₂ is formed by sputtering with atarget having an alloy composition of ZnGa₂. Thereafter, theabove-described sample was moved to an oxidation chamber of which theinside was kept in an ultrahigh vacuum of 1×10⁻⁸ Pa or less to performnatural oxidation by introducing Ar and oxygen. The natural oxidationtime was 10 seconds, the partial pressure ratio of Ar to oxygen was 1 to25, and the total gas pressure was 0.05 Pa. Then, the sample wasreturned to a film forming chamber, and a film of ZnGa₂ with a thicknessof 0.4 nm was formed. The above-described sample was moved to theoxidation chamber of which the inside was kept in an ultrahigh vacuum of1×10⁻⁸ Pa or less to perform natural oxidation and inductively coupledplasma oxidation by introducing Ar and oxygen. The natural oxidationtime was 30 seconds, and the inductively coupled plasma oxidation timewas 5 seconds. The partial pressure ratio of Ar to oxygen was 1 to 20,and the total gas pressure was 0.08 Pa.

(Characteristics of Example 2)

FIG. 12 is a diagram showing results of evaluation of themagnetoresistance effect of the magnetoresistance effect element ofExample 2. From FIG. 12, it was recognized that the resistance wasrapidly increased with the current of 1.2 mA and rapidly decreased withthe current of −1.3 mA. Therefore, the reversal current values are 1.2mA and −1.3 mA, respectively. It was also found that the area resistance(RA) was 0.64 Ω·μm² in a case where the magnetization directions of thefirst and second ferromagnetic metal layers were parallel to each other.The MR ratio was 50.9% in a case where the bias voltage was 1 V. Thistunnel barrier layer was confirmed as a spinel structure consisting of acubic structure from an electron beam diffraction image.

The MR ratio was calculated using the results shown in FIG. 12. FIG. 13is a diagram showing a decreasing rate of the MR ratio at the biasvoltage on the basis of the MR ratio at the low bias voltage as areference. In the drawing, the horizontal axis represents an appliedcurrent, and the vertical axis represents a decreasing rate of the MRratio on the basis of the MR ratio in a case where the bias voltage of0.001 V was applied. From FIG. 13, it was found that the voltage(V_(half)) at which the MR ratio decreased by half was 1.0 V or −0.9 V

Example 3

The production method is similar to that in the Example 1, but only thematerial for forming the tunnel barrier layer is different from that ofExample 1. A 0.5 nm thick film of CdGa₂ is formed by sputtering with atarget having an alloy composition of CdGa₂. Thereafter, theabove-described sample was moved to an oxidation chamber of which theinside was kept in an ultrahigh vacuum of 1×10⁻⁸ Pa or less to performnatural oxidation by introducing Ar and oxygen. The natural oxidationtime was 10 seconds, the partial pressure ratio of Ar to oxygen was 1 to25, and the total gas pressure was 0.05 Pa. Then, the sample wasreturned to a film forming chamber, and a film of CdGa₂ with a thicknessof 0.4 nm was formed. The above-described sample was moved to theoxidation chamber of which the inside was kept in an ultrahigh vacuum of1×10⁻⁸ Pa or less to perform natural oxidation and inductively coupledplasma oxidation by introducing Ar and oxygen. The natural oxidationtime was 30 seconds, and the inductively coupled plasma oxidation timewas 5 seconds. The partial pressure ratio of Ar to oxygen was 1 to 20,and the total gas pressure was 0.08 Pa.

(Characteristics of Example 3)

FIG. 14 is a diagram showing results of evaluation of themagnetoresistance effect of the magnetoresistance effect element ofExample 3. From FIG. 14, it was recognized that the resistance wasrapidly increased with the current of 1.5 mA and rapidly decreased withthe current of −1.5 mA. Therefore, the reversal current values are ±1.5mA, respectively. It was also found that the area resistance (RA) was0.67 Ω·μm² in a case where the magnetization directions of the first andsecond ferromagnetic metal layers were parallel to each other.

The MR ratio was calculated using the results shown in FIG. 14. FIG. 15is a diagram showing a decreasing rate of the MR ratio at the biasvoltage on the basis of the MR ratio at the low bias voltage as areference. In the drawing, the horizontal axis represents a biasvoltage, and the vertical axis represents a decreasing rate of the MRratio on the basis of the MR ratio in a case where the bias voltage of0.001 V was applied. From FIG. 15, it was found that the voltage(V_(half)) at which the MR ratio decreased by half was 1.1 V or −1.1 V.Moreover, a cross-section of the element was observed using an electronbeam transmission apparatus to evaluate the tunnel barrier layer, thefirst ferromagnetic metal layer and the second ferromagnetic metallayer, respectively, with the electron beam diffraction. This tunnelbarrier layer was confirmed as a spinel structure consisting of a cubicstructure from an electron beam diffraction image. Furthermore, it wasalso confirmed that the cubic-on-cubic structure was formed since any ofthe tunnel barrier layer, the first ferromagnetic metal layer and thesecond ferromagnetic metal layer shared the same direction of the basiclattice.

Example 4

The production method is similar to that in the Example 1, but only thematerial for forming the tunnel barrier layer is different from that ofExample 1. A film of 0.05 nm thick Mg/0.4 nm thick Mg_(0.5)Ga₂ wasformed by sputtering with a target of Mg and a target of an alloycomposition of Mg_(0.5)Ga₂. Thereafter, the above-described sample wasmoved to an oxidation chamber of which the inside was kept in anultrahigh vacuum of 1×10⁻⁸ Pa or less to perform natural oxidation byintroducing Ar and oxygen. The natural oxidation time was 10 seconds,the partial pressure ratio of Ar to oxygen was 1 to 25, and the totalgas pressure was 0.05 Pa. Then, the sample was returned to a filmforming chamber, and a film of 0.05 nm thick Mg/0.4 nm thick Mg_(0.5)Ga₂was formed. The above-described sample was moved to the oxidationchamber of which the inside was kept in an ultrahigh vacuum of 1×10⁻⁸ Paor less to perform natural oxidation and inductively coupled plasmaoxidation by introducing Ar and oxygen. The natural oxidation time was30 seconds, and the inductively coupled plasma oxidation time was 5seconds. The partial pressure ratio of Ar to oxygen was 1 to 20, and thetotal gas pressure was 0.08 Pa.

(Characteristics of Example 4)

FIG. 16 is a diagram showing results of evaluation of themagnetoresistance effect of the magnetoresistance effect element ofExample 4. From FIG. 16, it was recognized that the resistance wasrapidly increased with the current of 0.8 mA and rapidly decreased withthe current of −1.2 mA. Therefore, the reversal current values are 0.8mA and −1.2 mA, respectively. It was also found that the area resistance(RA) was 0.56 Ω·μm² in a case where the magnetization directions of thefirst and second ferromagnetic metal layers were parallel to each other.

The MR ratio was calculated using the results shown in FIG. 16. FIG. 17is a diagram showing a decreasing rate of the MR ratio at the appliedvoltage on the basis of the MR ratio at the low bias voltage as areference. In the drawing, the horizontal axis represents a biasvoltage, and the vertical axis represents a decreasing rate of the MRratio on the basis of the MR ratio in a case where the bias voltage of0.001 V was applied. From FIG. 17, it was found that the voltage(V_(half)) at which the MR ratio decreased by half was 1.1 V or −0.8 V.The MR ratio was 108% in a case where the bias voltage was 1V

(Composition and Structure Analyses of Example 4)

By comparing relative amounts using EDS, Mg:Ga was confirmed as 0.81:2.Structure analysis of the tunnel barrier layer was evaluated with anelectron diffraction image obtained using a transmission electronmicroscope. Through this method, the structure of the barrier layer wasexamined, and it was confirmed that there was no reflection from the{022} plane and the {111} plane shown in the ordered spinel structure.In addition, it was found that this barrier had a cubic structure inwhich the spinel structure was disordered.

Example 5

The production method is similar to that in the Example 1, but only thematerial for forming the tunnel barrier layer is different from that ofExample 1. A film of Mg_(x)Ga₂O₄ was formed by sputtering with a targetof Mg and a target of an alloy composition of Mg_(0.5)Ga_(z).

(Characteristics of Example 5)

The reversal current value was evaluated based on the measurementresults of the magnetoresistance effect. Moreover, the ratio of Mg andGa was obtained using EDS. FIG. 18 is a diagram showing a maximum valueof the reversal current and a concentration ratio of Mg to Ga in a casewhere a concentration of Ga is 2 in Example 5. From FIG. 18, it wasfound that the maximum value of the reversal current was the lowest in acase where a composition ratio of Mg was 0.9 to 1.25 when aconcentration of Ga is 2.

Example 6

The production method is similar to that in Example 1, but only themethod for forming the tunnel barrier layer is different from that ofExample 1. Film formation was performed on a substrate provided with athermal silicon oxide film using a magnetron sputtering method. As anunderlayer, 5 nm of Ta/3 nm of Ru was formed, and as a firstferromagnetic metal layer, [Co (0.24 nm)/Pt (0.16 nm)]₆/Ru (0.9 nm)/[Pt(0.16 nm)/Co (0.16 nm)]₄/Ta (0.2 nm)/FeB (1.0 nm) was formed in orderthereon. Next, a method of forming a tunnel barrier layer will be shown.A 0.5 nm thick film of MgGa₂ is formed by sputtering with a targethaving an alloy composition of MgGa₂. Thereafter, the above-describedsample was moved to an oxidation chamber of which the inside was kept inan ultrahigh vacuum of 1×10⁻⁸ Pa or less, and the substrate was cooledto −70 to −30 degrees. Then, natural oxidation was performed byintroducing Ar and oxygen. The natural oxidation time was 10 seconds,the partial pressure ratio of Ar to oxygen was 1 to 25, and the totalgas pressure was 0.05 Pa. Then, the sample was returned to a filmforming chamber, and a film of MgGa₂ with a thickness of 0.4 m wasformed. The above-described sample was moved to the oxidation chamber ofwhich the inside was kept in an ultrahigh vacuum of 1×10⁻⁸ Pa or less,and the substrate was cooled to −70 to −30 degrees. Then, naturaloxidation and inductively coupled plasma oxidation were performed byintroducing Ar and oxygen. The natural oxidation time was 30 seconds,and the inductively coupled plasma oxidation time was 5 seconds. Thepartial pressure ratio of Ar to oxygen was 1 to 20, and the total gaspressure was 0.08 Pa.

(Cross-Section Analysis of Example 6)

A volume ratio of the lattice-matched part (lattice-matched portion)with respect to the volume of the entire tunnel barrier layer wascalculated as described above using a cross-section transmissionelectron microscope (TEM) image and an image obtained by removingelectron beam diffraction spots in a direction other than a stackingdirection in an electron beam diffraction image obtained byFourier-transforming the TEM image and by then performing inverseFourier transformation.

FIG. 22 is a structural schematic diagram of a cross-section including adirection parallel to the stacking direction of Example 6. From thehigh-resolution cross-section TEM image obtained in Example 6, it wasfound that a size (width) of the film surface of the lattice-matchedpart of the tunnel barrier layer in a direction parallel thereto was 30nm or less in any part. 30 nm is about 10 times the lattice constant ofthe CoFe alloy that is the material of the first ferromagnetic metallayer and the second ferromagnetic metal layer, and mutual interferenceof the spin-polarized electrons in a direction perpendicular to thetunneling direction before or after coherent tunneling can be thought tobe intensified about 10 times the lattice constant.

FIG. 23 is a diagram showing the volume ratio of the lattice-matchedpart (lattice-matched portion) with respect to the volume of the entiretunnel barrier layer of Example 6 and characteristics of the element.FIG. 23(a) is a diagram showing element resistance (Rp) whenmagnetization directions of the first ferromagnetic metal layer and thesecond ferromagnetic metal layer are parallel to each other. FIG. 23(b)is a diagram showing element resistance (Rap) when magnetizationdirections of the first ferromagnetic metal layer and the secondferromagnetic metal layer are antiparallel to each other. FIG. 23(c) isa diagram showing a magnetoresistance ratio of the element. The Rp tendsto be reduced when the proportion of the lattice-matched part in whichthe tunnel barrier layer is lattice-matched to both of the firstferromagnetic metal layer and the second ferromagnetic metal layer is inthe range of 65% to 95%. Regarding this, in a case where the tunnelbarrier layer is completely lattice-matched, spin-polarized electronsinterfere with each other while passing through the tunnel barrierlayer, and thus it is thought that the electrons do not easily passthrough the tunnel barrier layer. In contrast, in a case where thelattice-matched parts in which lattices are matched partially exists,the interference of spin-polarized electrons during passing through thetunnel barrier layer is appropriately cut in a part in which latticesare not matched, and thus the spin-polarized electrons easily passthrough the tunnel barrier layer. As a result, it is thought that atendency of a reduction in the Rp is observed. At the same time, atendency of a slight increase in the Rap is observed when the proportionof the lattice-matched portion is in the range of 65% to 95%. Thisindicates that even when the magnetization directions of the firstferromagnetic metal layer and the second ferromagnetic metal layer areantiparallel to each other, the interference between domains is eased,and it is found that the spin-polarized electrons passing through thetunnel barrier layer are magnetically scattered.

Comparative Example 1

The production method is similar to that in Example 1, but only thematerial for forming the tunnel barrier layer is different that ofExample 1. A 0.45 nm thick Mg film was formed by sputtering with a Mgtarget. Thereafter, the above-described sample was moved to an oxidationchamber of which the inside was kept in an ultrahigh vacuum of 1×10⁻⁸ Paor less to perform natural oxidation by introducing Ar and oxygen. Thenatural oxidation time was 10 seconds, the partial pressure ratio of Arto oxygen was 1 to 25, and the total gas pressure was 0.05 Pa. Then, thesample was returned to a film-forming chamber, and a film of Mg with athickness of 0.4 nm was formed. Furthermore, the above-described samplewas moved to the oxidation chamber of which the inside was kept in anultrahigh vacuum of 1×10⁻⁸ Pa or less to perform natural oxidation andinductively coupled plasma oxidation by introducing Ar and oxygen. Thenatural oxidation time was 30 seconds, and the inductively coupledplasma oxidation time was 5 seconds. The partial pressure ratio of Ar tooxygen was 1 to 20, and the total gas pressure was 0.08 Pa.

(Characteristics of Comparative Example 1)

FIG. 19 is a diagram showing results of evaluation of amagnetoresistance effect of a magnetoresistance effect element ofComparative Example 1. From FIG. 19, it was recognized that theresistance was rapidly increased with the current of 2.5 mA and rapidlydecreased with the current of −5 mA. Therefore, the reversal currentvalues are 2.5 mA and −5 mA, respectively. It was also found that thearea resistance (RA) was 0.64 Ω·μm² in a case where the magnetizationdirections of the first and second ferromagnetic metal layers wereparallel to each other.

The MR ratio was calculated using the results shown in FIG. 19. FIG. 20is a diagram showing a decreasing rate of the MR ratio at the appliedvoltage on the basis of the MR ratio at the low bias voltage ofComparative Example 1 as a reference. In the drawing, the horizontalaxis represents a bias voltage, and the vertical axis represents adecreasing rate of the MR ratio on the basis of the MR ratio in a casewhere the bias voltage of 0.001 V was applied. From FIG. 20, it wasfound that the voltage (V_(half)) at which the MR ratio decreased byhalf was 0.45 V or −0.2 V. The MR ratio was 27% in a case where the biasvoltage was 1 V.

Comparison of Examples with Comparative Examples

Table 1 shows the examples and the comparative examples.

TABLE 1 RA MR Ratio EXAMPLES [Ω · μm²] [%@ 1 V] V_(half) [V] Jc [mA]EXAMPLE 1 0.6 64.8 1.05-1    1.0-1.05 EXAMPLE 2 0.64 50.9   1-0.91.2-1.3 EXAMPLE 3 0.67 51 ±1.0 ±1.5 EXAMPLE 4 0.56 108 1.1-0.8 0.8-1.2COMPARATIVE EXAMPLES COMPARATIVE 0.59 27 0.45-0.2  2.5-5   EXAMPLES 1

Comparison of Examples and Comparative Examples shows that any of MRratio, V_(half) and Jc is better for Examples. In Example 4, thereversal current values are 0.8 mA and −1.2 mA, respectively. Thereversal current is low in a case where the current flows in thepositive direction. However, it is preferable that the information isrewritten with the same current value when rewriting information in thedevice in terms of the design. That is, the element of Example 4 needs acurrent of at least 1.2 mA or larger for rewriting the information.Accordingly, the reversal current value of the device becomes lower in acase where a composition ratio of Mg is 0.9 to 1.25 when a concentrationof Ga is 2.

INDUSTRIAL APPLICABILITY

This invention is possible to apply to a magnetoresistance effectelement which is able to effectively carry out the magnetizationreversal due to the current and has more excellent symmetry of the MRratio for the bias voltage compared with the TMR element using MgO, aconventional tunnel barrier layer.

DESCRIPTION OF THE REFERENCE SYMBOLS

-   -   100: MAGNETORESISTANCE EFFECT ELEMENT    -   1: SUBSTRATE    -   2: UNDERLAYER    -   3: TUNNEL BARRIER LAYER    -   4: CAP LAYER    -   5: ELECTRODE LAYER    -   6: FIRST FERROMAGNETIC METAL LAYER    -   7: SECOND FERROMAGNETIC METAL LAYER    -   8: ELECTRODE PAD    -   71: CURRENT SOURCE    -   72: VOLTMETER

The invention claimed is:
 1. A magnetoresistance effect elementcomprising: a first ferromagnetic metal layer; a second ferromagneticmetal layer; and a tunnel barrier layer that is sandwiched between thefirst and second ferromagnetic metal layers, wherein the tunnel barrierlayer has a spinel structure represented by a composition formulaAGa₂O_(x) (0<x≤4), and an A-site is a non-magnetic divalent cation whichis one or more selected from a group consisting of magnesium, zinc andcadmium, and wherein the tunnel barrier layer comprises: at least onelattice-matched portion that is lattice-matched with both of the firstferromagnetic metal layer and the second ferromagnetic metal layer; andat least one lattice-mismatched portion that is not lattice-matched withat least one of the first ferromagnetic metal layer and the secondferromagnetic metal layer, and when viewed as an inverse Fouriertransform image in a stacking direction of a cross-sectional crystallattice image of the interface between the tunnel barrier layer and thefirst and/or the second ferromagnetic metal layer, a lattice-matchedportion is made up of a plurality of sequential, continuously-connectedlattice lines, and a lattice-mismatched portion is made up of aplurality of sequential, non-continuously-connected lattice lines and/orno lattice lines.
 2. The magnetoresistance effect element according toclaim 1, wherein a volume ratio of the lattice-matched portion withrespect to a volume of the entire tunnel barrier layer is 65% to 95%. 3.The magnetoresistance effect element according to claim 2, wherein thetunnel barrier layer has a disordered spinel structure.
 4. Themagnetoresistance effect element according to claim 3, wherein a crystallattice of either or both of the first ferromagnetic metal layer and thesecond ferromagnetic metal layer is matched with a crystal lattice ofthe tunnel barrier layer to form a cubic-on-cubic structure.
 5. Themagnetoresistance effect element according to claim 4, wherein acomposition ratio of the divalent cation and the Ga ion is within arange of 0.9 to 1.25:2.
 6. The magnetoresistance effect elementaccording to claim 3, wherein a composition ratio of the divalent cationand the Ga ion is within a range of 0.9 to 1.25:2.
 7. Themagnetoresistance effect element according to claim 2, wherein a crystallattice of either or both of the first ferromagnetic metal layer and thesecond ferromagnetic metal layer is matched with a crystal lattice ofthe tunnel barrier layer to form a cubic-on-cubic structure.
 8. Themagnetoresistance effect element according to claim 7, wherein acomposition ratio of the divalent cation and the Ga ion is within arange of 0.9 to 1.25:2.
 9. The magnetoresistance effect elementaccording to claim 2, wherein a composition ratio of the divalent cationand the Ga ion is within a range of 0.9 to 1.25:2.
 10. Themagnetoresistance effect element according to claim 1, wherein thetunnel barrier layer has a disordered spinel structure.
 11. Themagnetoresistance effect element according to claim 10, wherein acrystal lattice of either or both of the first ferromagnetic metal layerand the second ferromagnetic metal layer is matched with a crystallattice of the tunnel barrier layer to form a cubic-on-cubic structure.12. The magnetoresistance effect element according to claim 11, whereina composition ratio of the divalent cation and the Ga ion is within arange of 0.9 to 1.25:2.
 13. The magnetoresistance effect elementaccording to claim 10, wherein a composition ratio of the divalentcation and the Ga ion is within a range of 0.9 to 1.25:2.
 14. Themagnetoresistance effect element according to claim 1, wherein a crystallattice of either or both of the first ferromagnetic metal layer and thesecond ferromagnetic metal layer is matched with a crystal lattice ofthe tunnel barrier layer to form a cubic-on-cubic structure.
 15. Themagnetoresistance effect element according to claim 14, wherein acomposition ratio of the divalent cation and the Ga ion is within arange of 0.9 to 1.25:2.
 16. The magnetoresistance effect elementaccording to claim 1, wherein a composition ratio of the divalent cationand the Ga ion is within a range of 0.9 to 1.25:2.
 17. Themagnetoresistance effect element according to claim 1, wherein amagnetoresistance ratio is 100% or greater when applying a bias voltageof 1 V or higher at room temperature.
 18. The magnetoresistance effectelement according to claim 1, wherein at least one of the firstferromagnetic metal layer and the second ferromagnetic metal layer hasmagnetic anisotropy perpendicular to a stacking direction.
 19. Themagnetoresistance effect element according to claim 1, wherein at leastone of the first ferromagnetic metal layer and the second ferromagneticmetal layer is Co₂Mn_(1-a)Fe_(a)Al_(b)Si_(1-b) (0≤a≤1, 0≤b≤1).